In the field of data communications, the rate at which information is passed between computers, or between a computer and a high speed peripheral device, has been steadily rising. Traditionally, this function has been accomplished by cables made from copper wires. These cables may include multiple wires, each wire being a single conductor, bundled or in a ribbon configuration, coaxial cables, twisted pairs of wires, etc., and shielded in various ways.
Several factors appear to be driving the data communications industry to solutions that do not require cables constructed from copper wires. First, the size of cables and connectors with large numbers of parallel conductors is becoming unacceptably large. Second, the radio frequency power radiated by cables in which high speed signals are transmitted is difficult to control and the susceptibility of the wires to picking up radio frequency signals from external radio frequency noise sources is difficult to minimize. Third, the length of low-cost cable over which high-speed signals may be sent in a reasonably-priced copper cable technology is too short for applications currently contemplated.
For these reasons, low-cost, high-speed, compact, low-EMI, parallel fiber optic data interconnects have been a goal within the optoelectronics industry.
Because there has yet been no agreement within the industry regarding the various engineering and cost compromises that must be made when designing such data interconnects, no significant standard has arisen. For example, unlike this invention, most products, prototypes or proposals for high-speed parallel fiber optic interconnects use standard 62.5/125 micrometer multimode optical fiber.
Among the features of a high-speed, parallel, fiber optic, data communications link that are foreseen as needed by the computer industry in the near future are:
1) speeds from 200 megabits per second (Mbs) to 1000 Mbs per channel, with aggregate speeds from 150 megabytes per second (MBs) to 2 gigabyte per second (GBs), PA1 2) cable lengths from approximately ten meters to several hundreds of meters, PA1 3) cables offering low weight, high flexibility, and relatively high density as measured by the average cross-sectional area per channel, PA1 4) connectors of sufficiently small size that their impact on the design of connector panels and on the use of edge real estate on interface cards is reasonable, PA1 5) low radiated electromagnetic power, low susceptibility to electromagnetic interference, and avoidance of ground loops, PA1 6) hot pluggability (the possibility for reconfiguring a computer while running), and PA1 7) low cost to produce. PA1 1. Inexpensive molded thermoplastic parts are used for optical packaging elements. This approach has been previously employed with a single-channel optoelectronic link by making the housing, ferrule, bore, and lens of the optical subassembly by injection molding. In the present work the approach is extended to a parallel optical link. PA1 2. It has recently been demonstrated that high yield and high uniformity vertical cavity surface emitting laser ("VCSEL") monolithic array chips and MSM-PD photoreceiver array chips are manufacturable. These OE chips are employed in the present invention, not only because of their low inherent cost, but because the VCSEL's optical beam emission is relatively collimated (compared to edge-emitting laser or LED alternatives) making it easier to align the VCSEL array chip with the optics of the transmitter module, and because the MSM-PD has a large light sensitivity area (compared to high speed pin PDs) making the photoreceiver array chip easier to align to the optics in the receiver module. PA1 3. Plastic-package leadframe technology, adapted from the high volume IC industry, is used for this POI because optical alignment features of sufficient precision can be co-molded with the module housing, and because it is one of the lowest cost high lead count multichip carriers available. PA1 4. A unique plastic-molded optical coupler array is used for coupling light from the optical transmitter chip to the fibers and light from the fibers to the receiver chip. A 90 degree bend in the optical path can be incorporated in the optical coupler in order to maintain the plane of the cable parallel to the plane of the circuit card and chip sets. PA1 5. Large-core, plastic-clad fiber (200/230 micrometers diameter, wherein the first term refers to the core and the second refers to the cladding about the core; it should be understood, however, that the invention is applicable with other core/cladding combinations as well) is used to make a fiber ribbon cable. The core diameter of each fiber is large enough to permit simplified, passive alignment, yet not so large that the fiber ribbon cable has inadequate flexibility. In order to maintain low cost, optical ribbon jumper cables are made by fabricating the connector body directly on the parallel fibers of the fiber ribbon cable. Lowest cost is achieved through the use of step-index fiber, which is appropriate for short-distance applications, e.g., up to about 50 meters. For those applications requiring longer distances, large-core graded index fiber may be used. PA1 6. Alignment of the fiber to the optically active chips is usually a difficult and expensive task in optoelectronic packaging. In order to achieve low-cost packaging, alignment in the present application (in the preferred embodiment) is carried out with the aid of special but easily fabricated locating features provided in the package, which permit positioning of the optically active chips and the optical coupler array by simple mechanical reference to these locating features. This method avoids more expensive "active" alignment (i.e., searching for the optimum position with the chips activated) or "passive" alignment based on high precision micro-features which are difficult to fabricate. PA1 7. A protective shell is used to control the connector approach to the optical coupler. The shell provides angled lead-in and alignment keys to ensure that the connector enters the coupler at or near the correct position and at or near the correct angle. The shell, in conjunction with latch features on the cable connector, gives a slight pre-load force on the optical fibers to the optical coupler, and provides a strain-relief release in case excess pull-out force is exerted on the cable, thus protecting the cable, module and circuit card.
Such features are difficult to attain with conventional copper cable, except for very short interconnects. Serializing parallel line data interfaces onto high speed single fiber optic lines has been shown to be practical for distances of greater than about 20 meters but only for data rates .ltoreq.100 megabytes/sec (Mbs). Also, the functions of serialization and deserialization, and associated clocking of the data often add too much time delay to the transmission.
Since the cable, connectors, transmitters, receivers, and packaging associated with optoelectronic links have historically been relatively expensive, it has proven difficult to achieve optoelectronic links of low enough cost to compete with copper cable.
Previously described parallel line optical interconnects have adapted the optoelectronic chips and the optical packaging from high speed serial data link technology, resulting in excessively expensive parallel optical interconnects. One specific drawback of the prior systems is the use of components which require precision alignment (.ltoreq.5 .mu.m tolerance in alignment) between optoelectronic chips and optical elements, and thus a package assembly procedure requiring fine manipulations of small parts and even electrical activation of the device to aid in its alignment.
A promising approach to this problem has been previously suggested and involves the abandonment of the serialization-deserialization, single-fiber scheme used in previous computer optoelectronic data links in favor of a parallel link. Here an array of light sources is connected by a multi-fiber "ribbon" cable to an array of detectors, so that the difficulties associated with ultra high speed sources, detectors, circuits, and packaging are avoided, and the necessity for serialization/deserialization circuits is bypassed. However, this method requires the availability of relatively inexpensive (produced with high yields) detector and light-source array chips, low-cost means of aligning these chips to the fiber cable, and low-cost array connectors and cabling. In addition, special efforts must be made to keep the packaging costs of the transmitter and receiver subassemblies as low as possible.
A parallel optical interconnect ("POI") (which includes an array of optical links, each comprising a transmitter, a fiber and a receiver) in accordance with the present invention and described below achieves the desired performance goals at costs which are considerably lower than those associated with alternative POI. This result is achieved by a unique combination of components chosen for their ability to be packaged with loose alignment tolerances as well as their inherent low cost, together with module and cable components chosen for their ability to take advantage of the looser alignment tolerances to dramatically lower assembly cost, as well as their inherent low intrinsic cost. The salient features of this unique POI are:
In accordance with the invention, a 2 byte wide POI operating at 500 Mb/s per line utilizing 20 parallel lines with a digital logic interface was designed and built based on the features listed above. A channel pitch of 500 micrometers was adopted. A duplex parallel optical link was designed, comprising a transmitter module and a receiver module at each end of two optical ribbon jumper cables, i.e., the link included two transmitter modules and two receiver modules. The link was designed for either synchronous or asynchronous operation. Links having different speeds, number of channels, pitch, fibers (e.g., standard 50/125 or 62.5/125 micrometer multimode communications fiber instead of 200/230 micrometer fiber), optical connectors, operating wavelength, and I/O interfaces could easily be designed using the same basic principles described here. Also, instead of a separate transmitter module and receiver module at each end of the cable, a transceiver package which included both transmitter and receiver components could easily be designed.